This invention relates to crystal mounting structures. In particular this invention relates to crystal mounting structures that reduce mechanical stresses in a piezoelectric quartz crystal resonator.
FIG. 1 shows a representative cross-sectional diagram of a typical prior art piezoelectric quartz crystal resonator (10). A package base (20), which is typically metallic but may also be glass or ceramic, supports a piezoelectric quartz resonator element (30) which includes two electroded surfaces (the electrodes are shown as elements 32 and 34) supported between two fixed mounting posts (36) as shown.
The piezoelectric quartz crystal resonator (10) includes a thin piece of quartz (30), typically only a few thousandths of an inch thickness having a predetermined cut orientation (such as an AT-cut or GT-cut as they are known in the art). The cut orientation, and other physical dimensions and the electrode dimension establish the resonant frequency of the resonator (10). The piezoelectric quartz resonator element (30) typically has a thermal expansion coefficient of anywhere between 5 and 30 parts per million per degree centigrade although values between 10 and 20 parts per million per degree centigrade are more typical. The package base (20) on the other hand, which is typically metallic, has a thermal expansion coefficient of between 2 and 10 parts per million per degree centigrade.
In fabricating a crystal resonator (10) as shown in FIG. 1, the electrodes (32 and 34) on the quartz crystal plate (30) are attached to the mounting posts (36) by an appropriate adhesive. Conductive epoxy or solder may be used to attach the mounting posts (36) to the electrodes (32 and 34). To cure the adhesive used to join the mounting posts (36) to the electrodes (32 and 34), or to flow metal-based solders to join the posts to the electrodes, the quartz crystal resonator (10) usually must be heated to an elevated temperature. When heated the package base and quartz crystal plate expand, albeit at different rates and when cooled after the adhesive sets, the mechanical joints existing between the mounting posts create mechanical stresses in the quartz (30) because of the differences in thermal expansion coefficients. Since the thermal expansion coefficients of the package base (20) and the quartz crystal plate (30) are different, the physical dimensions of the pieces will change as their temperature shifts.
Alternate schemes of mounting the quartz crystal resonator (30) to a package base (20) include using a quartz material for the package base (20) which would have a substantially identical thermal expansion coefficient. The cost of using quartz for the package base creates other problems including expense and hermetically sealing the crystal resonator (10).
A crystal mounting structure which reduces mechanical stresses on the piezoelectric quartz crystal resonator at a reasonable cost would be an improvement over the prior art.